Many Orthodox Christians, do not understand why we fast. This is why we as a people fast so poorly. So many Orthodox Christians barely follow the fast or follow the fasts when they’re convenient, forget to fast, things like this.
We fast because of what the Scripture tells us today.
We fast because of this famous oft repeated saying of our Lord Jesus Christ:
“This kind goeth not out but by prayer and fasting” 
We also fast because of a stronger reason our Lord gives just before this.
The apostles come to Him after He had healed the demoniac boy. They couldn’t heal him. They didn’t have the faith to heal him. So they wondered why? And He answered their question as to why they could not heal the boy:
“Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. 21 Howbeit this kind goeth not out but by prayer and fasting. “ 
The mustard seed is a very small seed, very pungent, very strong in flavor. It flavors the entire dish. So our faith must be like that mustard seed. What He is saying, what the Fathers tell us, is that faith like this comes from becoming good, from pleasing God, and living according to the Commandments.
So that when we say that we have weak faith, we are saying at the same time that we are sinners, that we don’t do the law of God at all times.
Perhaps we don’t even place the law of God as a priority in our lives, because the doing comes first from the desiring to do and the planning to do.
All these things: To not desire to do the will of God, to not plan to do it, to not be able to do it: These are all from weak faith.
So the kind of faith that can cast out a demon from a boy, comes only by living according to the Gospel.
Now, this seems impossible to us. How can we do such things?
We do such things bit by bit, by prayer and by fasting. That is why fasting is so important, and it’s always joined with prayer.
And that is why we are so mediocre as Christians – we pray very little and we don’t fast with much attention. And that’s why we’re weak. But we have great things to be done. We are supposed to cast out demons.
Now, the demons in this boy cast him into fire and into water. And this is where it is important to identify ourselves with this boy. Remember, read the Scriptures and see what they say about you. It’s not going to really help you a whole lot to know what they say about other people. You need to know how to help yourself. You need to know how to become good, with God helping you, because you can’t make anybody else good. You can’t decide for anyone else to follow the Gospel. You have to decide for yourself to follow the Gospel and desire the Gospel above everything else. Reading the Scriptures should always be instructive for you in what you should do and what you shouldn’t do, how you should change, what you should keep doing, what kind of person you are, what kind of person you aren’t. It’s all there in the Gospel.
Can anyone identify with the man with weak faith? Has there ever been some thing that’s been difficult for you that you pray to God for but you don’t really quite believe but it can really be changed?
Look what the Lord called him. He said, “O faithless and perverse generation, how long shall I be with you? How long shall I suffer you?”  The Lord was both rebuking the man and the Jewish people and us for our lack of faith.
And remember, lack of faith is because of lack of righteousness. I told you many times, it’s always about morality.
Christian life is to become moral, to become good as God is good. Then we can know Him and understand Him. Until then we can only talk about Him and really have no idea what we’re talking about. We have to be, in order to understand; we have to become righteous.
This fire and the water that the demon cast the boy into represent various sins. Fire is hot sins, concupiscence and anger and those sorts of things. The Fathers say water represents it confusion and worldly cares.
Has anybody ever felt confused? Have you ever had times in your life when, you don’t really know what the next thing to do is, how to fix this problem? It happens to me all the time. I’m convinced, from reading the lives of the saints and from talking with people who are much more righteous than myself that it should happen to all of us all the time. It’s part of the human condition. And it’s the part that only comes out by prayer and by fasting.
The sins of desire and the sins of weakness of will, weakness of mind — these only come out by prayer and fasting.
But along with prayer and fasting must be faith as the mustard seed. And the faith is from doing. The faith is from becoming. There is no way of escaping it.
I’ve told you many times before, it’s my belief that the greatest heresy of all time is the idea that one can have salvation without labor. It is a relatively new heresy. It’s so new it wasn’t even spoken of really except obliquely in the Ecumenical Councils. The idea that one can have salvation without labor is a horrible idea because it’s false.
The man says that the boy is a lunatic, and he begs the Lord for help. A lunatic is someone who goes crazy at certain phases of the moon, and the reason why they went crazy at these phases of the moon is because the demons who inhabited these poor souls would stir them up at certain seasons so that people would associate their sickness with the moon which God created, so they would think it must be God doing this because the moon becomes full and these people become crazy. It was a way of trying to blaspheme the Creator through hurting those He created and slandering Him.
But the boy isn’t a lunatic. There’s no such thing as a lunatic. He was possessed by demons because of the weakness of his father.
How many things are wrong in our lives and we don’t know the reason? We think we know the reason. But we don’t know it because we don’t have the wisdom to see it. Wisdom comes from righteousness. Until we become righteous, we can’t see the truth about things. We only see darkly about things, and we make a lot of mistakes about cause and effect. The only solution for us as Christians is to struggle to follow the law of God. There’s not one thing you must do. You must do all of it.
Now, there are things that are part of this life. Fasting and prayer, are critical, not as an end, but a means – so that we would be able to cast out the fire and the water in our souls.
The Lord gives us all the capability to do it, but we must struggle to do it. So we must pray and fast and struggle to follow the Commandments. And little bit by little bit, some of that fire will be quenched; some of that water will be drained from our soul, and we will see things more clearly, and we will have these things cast out of us because of our great efforts and because of God’s Grace.
So, brothers and sisters, why do we fast? Because it’s absolutely necessary.
And it is no wonder most of the Christian world doesn’t fast. It’s critically important to the spiritual life that we fast and pray. It makes perfect sense to me why most of the Christian world, even the Orthodox Christian world, doesn’t fast, because it’s critically important.
So it is with us, but not fasting because it is an arbitrary rule, but in order to cast out the fire and the water in our souls. Amen.
SOURCE: St Nicholas Orthodox Church